Sensor and Method for Measuring Respiratory Gas Properties

ABSTRACT

A sensor and method for measuring respiratory gas properties are presented. A thermal conductivity sensor is used to measure the thermal conductivity of a gas with unknown composition and/or mass flow rate at different temperatures. The measured thermal conductivities at different temperatures are compared with known thermal conductivities of gases at different temperatures. In an exemplary application the sensor and method are installed in a tube to determine a mass of respiratory air flowing through the tube and a concentration of CO 2  therein.

TECHNICAL FIELD

The present disclosure generally relates to a sensor and method formeasuring gas properties, and more particularly, to a respiratory gassensing system and method for measuring the concentration of carbondioxide in a respiratory gas.

BACKGROUND

In the medical field, various diagnostic and therapeutic devices requirean analysis of gases within the air that is inhaled and/or exhaled by apatient. Such devices include spiroergometers, breathing trainers,respirators, and anesthesia machines. Due to high complexity and cost ofsuch devices their use is usually limited to medical applications orniche applications such as performance diagnostics in professionalsports.

Spirometry is the most common of the pulmonary function tests (PFTs),measuring lung function, specifically the amount (volume) and/or speed(flow) of air that can be inhaled and exhaled. Spirometry is animportant tool used for generating pneumotachographs, which are helpfulin assessing conditions such as asthma, pulmonary fibrosis, cysticfibrosis, and COPD.

Spirometers are known which use differential pressure sensors,ultrasonic sensors, turbine wheel sensors and hot-wire anemometers. Anexemplary spirometer with replaceable flow tube is disclosed in U.S.Pat. No. 8,753,286 which is incorporated by reference in its entirety.

A precise and direct method for determining metabolic activity in humansis the analysis of the respiratory gases, in which the concentration ofoxygen and carbon dioxide in the breath as well as the volume flow ofthe breath are determined. Metabolic values such as, for example, therespiratory quotient (RQ) can be calculated on the basis of themeasurements. RQ is the ratio of the amount of exhaled carbon dioxide tothe amount of oxygen taken in. To determine these gas quantities,various parameters are measured. The flow volume of the respiratory gasis determined using a sensor which measures the flow velocity of therespiratory gas by measuring the ultrasound travel time, for example.Various respiratory volumes can be derived through the integration ofthe volume flow over time. In known spiroergometry (or ergospirometry)devices, a gas sample is also suctioned off from the main respiratoryflow and fed to the sensor system contained in the device. The sensorsystem typically contain chemical analysis sensors. As a result, theconcentrations of oxygen and carbon dioxide upon inhaling and exhalingcan be determined. The respective values of the gas concentrationsdiffer substantially between inhaling and exhaling. Using the previouslydetermined respiratory volume, the gas quantities that were converted bythe body can be calculated from the concentrations. An exemplary userunit for determining output parameters from breath gas analyses isdisclosed in U.S. patent application publication US2012/0226180 which ishereby incorporated by reference.

A problem with conventional devices is that they are rather large,expensive, and energy-consuming, making them not suitable forbattery-powered, mobile use.

SUMMARY

A respiratory gas sensing system is presented. The sensor uses a thermalconductivity sensor which is disposed within a respiratory flow path. Aprocessing module is operatively connected to the thermal conductivitysensor. The processing module measures a first thermal conductivity ofthe respiratory gas at a first temperature and measures a second thermalconductivity of the respiratory gas at a second temperature. The secondtemperature is higher than the first temperature. The processing modulethen determines a concentration of carbon dioxide within the respiratorygas in response to the measured first thermal conductivity and themeasured second thermal conductivity. The processing module may useadditional inputs in determining the concentration of carbon dioxidebeyond the measured first and second thermal conductivity.

The thermal conductivity sensor may include a first thermal conductivitysensing element and a second thermal conductivity sensing element. Inthat case, the processing module may be configured to operate the firstthermal conductivity sensing element at the first temperature and tooperate the second thermal conductivity sensing element at the secondtemperature.

The respiratory gas sensing system may be arranged within a tube throughwhich the respiratory gas flows such that the first thermal conductivitysensing element and the second thermal conductivity sensing element arearranged in a plane substantially parallel to a longitudinal extensionof the tube. This arrangement is advantageous if thermal cross-talkbetween the first and the second thermal conductivity sensing element isdesired. That may be the case to reduce the overall energy required toheat the first and the second thermal sensing element due to theirarrangement in which heating one sensing element affects the temperatureof the other sensing element.

Alternatively, the first thermal conductivity sensing element and thesecond thermal conductivity sensing element may be arranged in a planesubstantially perpendicular to a longitudinal extension of the tube.That arrangement may be preferred to reduce thermal cross-talk betweenthe sensing elements.

The thermal conductivity sensing element may be a meander-shapedresistive wire forming part of an analog or digital measuring circuit.The measuring circuit may e.g. utilize a Wheatstone bridge. If twothermal conductivity sensing elements are used the first sensing elementmay be part of a measuring circuit (e.g. a first Wheatstone bridge) andthe second sensing element may be part of a second measuring circuit(e.g. a second Wheatstone bridge).

The first and the second thermal conductivity sensing elements may bearranged in close proximity to one another. In particular, the thermalconductivity sensing elements may be arranged on a membrane within anarea of less than 4 mm².

As described, the respiratory gas sensing system may use two differentthermal conductivity sensing elements which operate simultaneously attwo different temperatures. Alternatively, the respiratory gas sensingsystem may use a single thermal conductivity sensor which operates, overtime, at two or more different temperatures. More specifically, theprocessing module may be configured to alternately operate the thermalconductivity sensing element at the first temperature and at the secondtemperature. If the thermal conductivity sensing element is part of aWheatstone bridge the different temperatures of the thermal conductivitysensing element may be affected by selectively changing a resistor valueof a resistor in the Wheatstone bridge.

Within a larger device the respiratory gas sensing system may bearranged within a longitudinally elongated tube through whichrespiratory gas flows. The tube may have a proximal end and a distalend. The proximal end may be connected to a breathing mask which can beworn by a subject. In such a configuration a first temperature sensingelement may be provided laterally spaced apart from the thermalconductivity sensor on either the proximal or the distal side of thethermal conductivity sensing element. Alternatively, two temperaturesensing elements may be provided, one arranged on a proximal side of thethermal conductivity sensor and a second temperature sensing elementarranged on a distal side of the thermal conductivity sensor.

The sensor may use a have first temperature of less than 200° C. and asecond temperature of more than 200° C.

Measuring the thermal conductivity of a respiratory gas at two differenttemperatures may be sufficient to determine the concentration of CO₂therein, even if the humidity and/or flow of the respiratory gas isunknown. However, more accurate determination of CO₂ concentration maybe accomplished by using measurements at more than two differenttemperatures. For example, the respiratory gas sensing system maymeasure the thermal conductivity of a respiratory gas at three differenttemperatures and determine the concentration of carbon dioxide withinthe respiratory gas in response to all three measured thermalconductivities. In particular, the respiratory gas sensing system mayoperate at a first temperature which is less than 150° C., a secondtemperature between 150° C. and 250° C., and a third temperature of morethan 250° C.

To operate simultaneously at three different temperatures the thermalconductivity sensor may comprise a first thermal conductivity sensingelement, a second thermal conductivity sensing element, and a thirdthermal conductivity sensing element. The processing module may then beconfigured to operate the first thermal conductivity sensing element atthe first temperature, to operate the second thermal conductivitysensing element at the second temperature, and to operate the thirdthermal conductivity sensing element at the third temperature.Structurally, the third thermal conductivity sensing element may bearranged centrally between the first thermal conductivity sensingelement and the second thermal conductivity sensing, i.e. the hottestthermal conductivity sensing element may be arranged in between twothermal conductivity sensing elements which operate at lowertemperatures.

Alternatively, the sensor may utilize pairs of sensing elements thatoperate at the same temperature. For example, the thermal conductivitysensor may use two outer thermal conductivity sensing elements, twoinner thermal conductivity sensing elements and one central thermalconductivity sensing element. The sensor in that case may have asymmetrical structure in which the two inner thermal conductivitysensing elements are arranged inwardly of the two outer thermalconductivity sensing elements and the central thermal conductivitysensing element is arranged between the two inner thermal conductivitysensing elements. In that configuration the two outer thermalconductivity sensing elements may operate at the first temperature andthe inner thermal conductivity sensing element may operate at the secondtemperature. The central thermal conductivity sensing element mayoperate at a third temperature which is higher than the secondtemperature.

The respiratory gas sensing system may operate at absolute temperatures.That is, the first temperature and the second temperature may bepredetermined absolute temperatures. Alternatively, the respiratory gassensing system may operate at relative temperatures above ambienttemperature or above a temperature of the respiratory gas.

The sensor as described above may be utilized in various devices, andmay in particular be used in medical applications. For example, thesensor may be used within a spiroergometer having a tube with a proximalend and a distal end. A breathing mask may be connected to the proximalend of the tube and the sensor may be arranged within the tube. Thecompact form and low power consumption of the disclosed respiratory gassensing system enables spiroergometers in which the breathing mask andthe sensor are formed as a portable, battery powered device which isworn by a subject while exercising.

To improve measurement accuracy it may be desirable to measure thecarbon dioxide concentration in the respiratory gas while it is notmoving. This may be accomplished by providing a spiroergometer in whichthe tube comprises a first branch and a second branch. A trap sectionfor exhaled respiratory gas may be formed between two flow controlvalves in the second branch, and the respiratory gas sensing system mayutilize a thermal conductivity sensor arranged within the trap sectionof the second branch.

A respiratory flow control system may have a tube having a proximal endand a distal end. A breathing mask may be connected to the proximal endof the tube and be worn by a subject. A thermal conductivity sensor maybe arranged within the tube. The tube may comprise a first branch and asecond branch. A flow of respiratory gas through the first branch may becontrolled by a first flow control valve arranged within the firstbranch. A flow of respiratory gas through the second branch may becontrolled by a second flow control valve arranged within the secondbranch. The first flow control valve and the second flow control valvemay be controlled by the processing module in response to theconcentration of carbon dioxide determined by the processing module.

For anesthetic applications such a respiratory flow control system maybe used in a configuration where the tube comprises a first branchhaving an open distal end and a second branch having a distal endconnected to a reservoir comprising an anesthetic gas.

The respiratory flow control system may determine a respiratory volumeflow in response to measuring a respiratory mass flow and theconcentration of carbon dioxide. A method for analyzing a respiratorygas may be based on providing a thermal conductivity sensor within arespiratory flow path. The method may then use the steps of measuring afirst thermal conductivity of the respiratory gas at a first temperatureand measuring a second thermal conductivity of the respiratory gas at asecond temperature. Finally, the method may determine a concentration ofcarbon dioxide within the respiratory gas in response to the measuredfirst thermal conductivity and the measured second thermal conductivity.Determining the concentration of carbon dioxide may in particular bebased on comparing the measurements with data contained within a look-uptable. Determining the concentration of carbon dioxide within therespiratory gas may thus comprise comparing the first measured thermalconductivity and the second measured conductivity with known thermalconductivities of gases at different temperatures.

For improved measurement accuracy the thermal conductivity sensor may beprovided within a trap for exhaled air and the first thermalconductivity and the second thermal conductivity may be measured whileexhaled air is trapped, i.e. while the exhaled air is not flowing.

More generically, a method for measuring a property of a mixture ofgases is presented. The method includes providing one or more thermalconductivity sensing elements within the mixture of gases. Heating poweris applied to one of the one or more thermal conductivity sensingelements and controlled to maintain a selected first temperature. Afirst voltage and/or first power required to maintain the firsttemperature is measured. Further, heating power is applied to one of theone or more thermal conductivity sensing elements and controlled tomaintain a selected second temperature. A second voltage and/or secondpower required to maintain the second temperature is measured. Theconcentration of at least one gas contained in the mixture of gases isdetermined in response to the measured first voltage and/or first powerand the measured second voltage and/or second power.

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary respiratory gassensing system.

FIG. 2 is a schematic illustration of an alternative exemplaryrespiratory gas sensing system.

FIG. 3 is a schematic illustration of an exemplary spiroergometer.

FIG. 4 is a detailed schematic view of a thermal conductivity sensor.

FIG. 5 is a plot showing the correlation of the temperature of a thermalconductivity sensing element and the voltage applied thereto whensurrounded by different gases, the gas temperature being 20° C. withoutmovement of the gas.

FIG. 6 is a plot showing the correlation of the temperature of a thermalconductivity sensing element and the voltage applied thereto whensurrounded by different gases, the gas temperature being 20° C., withand without movement of the gas.

FIG. 7 is a schematic illustration of a breathing training device.

DETAILED DESCRIPTION

An exemplary respiratory gas sensing system 100 is schematicallyillustrated in FIG. 1. The sensing system 100 uses a thermalconductivity sensor 110 which is arranged within a tube 101 throughwhich a subject inhales and/or exhales. The tube 101 establishes arespiratory flow path 105 from a proximal end 102 to a distal end 103 oftube 101. The tube 101 may carry only a portion of the respiratory gasinhaled and/or exhaled by the subject. A processing module 120 isoperatively connected to the thermal conductivity sensor 110. Theprocessing module 120 measures a first thermal conductivity of therespiratory gas present within the tube 101 at a first temperature and asecond thermal conductivity of the respiratory gas at a second, highertemperature. The processing module then determines a concentration ofcarbon dioxide within the respiratory gas in response to the measuredfirst thermal conductivity and the measured second thermal conductivity.The processing module may use a processor 130 having an internal memory133 to perform the determination of carbon dioxide concentration.

The thermal conductivity sensor 110 may have a first thermalconductivity sensing element 111 which operates at the first temperatureand a second thermal conductivity sensing element 112 which operates atthe second temperature. The thermal conductivity sensing elements may beelectrically connected within electronic measuring circuits. Theelectronic measuring circuits are configured to generate an outputsignal which changes with the thermal conductivity of the airsurrounding the sensing elements 111,112. The sensing elements 111,112are PTC elements, that is they have a positive temperature coefficientof resistance. Various forms of electronic measuring circuits are knownto be used to evaluate the resistance of a PTC element and may be usedin combination with the sensing elements 111,112.

One exemplary electronic measuring circuit is a Wheatstone bridgecircuit. As shown in FIG. 1, the first thermal conductivity sensingelement 111 is part of a first Wheatstone bridge 140. The first thermalconductivity sensing element 111 may be a meander-shaped resistive wirewhich is electrically connected in series with a first resistor R₁. Thefirst resistor R₁ and the first thermal conductivity sensing element arewired in parallel to a second resistor R₂ and a third resistor R₃,jointly forming the Wheatstone bridge 140. The voltage across theWheatstone bridge 140 forms the input of a first amplifier 122. Theoutput of the first amplifier 123 powers the Wheatstone bridge 140.

The first amplifier 122 may be an operational amplifier having a highamplification factor. In that case the amplifier's output voltage U₁will assume a steady state when its input is essentially zero, i.e. whenthe voltage across the Wheatstone bridge is essentially zero. This isthe case when

$\frac{R_{2}}{R_{3}} = \frac{R_{1}}{R_{111}}$

with R₁₁₁ being the resistance of the first thermal conductivity sensingelement 111. The resistance of the first thermal conductivity sensingelement 111 is temperature-dependent and may increase with temperature.

R ₁₁₁ =f(T ₁₁₁)

With T₁₁₁ being the temperature of the first thermal conductivitysensing element 111.

The voltage across the first Wheatstone bridge 140 thus becomes zerowhen the first thermal conductivity sensing element 111 assumes aTemperature T₁₁₁ so that

$\frac{R_{2}}{R_{3}} = \frac{R_{1}}{f\left( T_{111} \right)}$

The power needed to heat the first thermal conductivity sensing element111 to its first operating temperature T₁₁₁ depends on the thermalconductivity of the respiratory gas surrounding the sensing element 111.Consequently, the output voltage U₁ of the first amplifier 122 is ameasure of the thermal conductivity of the respiratory gas surroundingthe first thermal conductivity sensing element 111. The output of thefirst amplifier 123 is operatively connected to a first A/D input 131 ofa processor 130 which determines of the concentration of carbon dioxidewithin the respiratory gas.

The second thermal conductivity sensing element 112 is electricallyconnected within a second Wheatstone bridge 150. In particular, thesecond thermal conductivity sensing element 112 is connected in serieswith a fourth resistor R₁′. The second thermal conductivity sensingelement 112 and the fourth resistor R₁′ are connected parallel to afifth resistor R₂′and a sixth resistor R₃′. The voltage across thesecond Wheatstone bridge 150 provides an input for a second amplifier124. The output of the second amplifier 125 powers the second Wheatstonebridge 150 and is operatively connected to a second A/D input 132 of theprocessor 130. The resistor values of R₁′, R₂′, R₃′ are selected suchthat the second thermal conductivity sensing element 112 operates at thesecond temperature T₁₁₂.

The processor 130 may be an integrated microcontroller. The processor130 may read (sensor) inputs, control outputs, manage sensor signals,interpret the signals as computer data, communicate with other controlsystems and remote computer systems, receive user input, and instructactuators to control flow control valves, as described below.

The processor 130 may run computer programs that may comprise orderedlistings of executable instructions for implementing logical functions.The computer programs may be stored in or on computer-readable medium133 residing on or accessible by the processor 130. The computerprograms can be embodied in any computer-readable medium for use by orin connection with an instruction execution system, apparatus, ordevice, such as a computer-based system, processor-containing system, orother system that can fetch the instructions from the instructionexecution system, apparatus, or device, and execute the instructions. Inthe context of this document, a “computer-readable medium” can be anymeans that can contain, store, communicate, propagate or transport theprogram for use by or in connection with the instruction executionsystem, apparatus, or device. The computer-readable medium can be, forexample, but is not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semi-conductor system, apparatus, device,or propagation medium. More specific, although not inclusive, examplesof the computer-readable medium would include the following: anelectrical connection having one or more wires, a portable computerdiskette, a random access memory (RAM), a read-only memory (ROM), anerasable, programmable, read-only memory (EPROM or Flash memory), anoptical fiber, and a portable compact disk read-only memory (CDROM).

The respiratory gas sensing system 100 as shown in FIG. 1 uses a thermalconductivity sensor 110 with two thermal conductivity sensing elements111,112. The respiratory gas sensing system 100 can simultaneouslydetermine the thermal conductivity of the respiratory gas at twodifferent temperatures. In some applications it may suffice to determinethe thermal conductivity of the respiratory gas sequentially at twodifferent temperatures. An exemplary such system is shown in FIG. 2.

The respiratory gas sensing system 200 shown in FIG. 2 uses a thermalconductivity sensor 210 which contains a single thermal conductivitysensing element 211. The thermal conductivity sensor 210 is arranged ina tube 202 which is filled with a respiratory gas so that the thermalconductivity sensor 210 is surrounded by the respiratory gas that is tobe analyzed. The thermal conductivity sensing element 211 may be ameander shaped resistive wire with a positive temperature coefficient(PTC). That it, the thermal conductivity sensing element 211 may be madeof a material that experiences an increase in electrical resistance whenits temperature is raised.

The thermal conductivity sensing element 211 is connected to a measuringcircuit, e.g. a Wheatstone bridge 240. The Wheatstone bridge 240 may beformed by a first resistor R₂₁ in series with the thermal conductivitysensing element 211 and a second resistor R₂₂ and third resistor R₂₃ inparallel to the first resistor R₂₁ and the sensing element 211. TheWheatstone bridge 240 may be powered by the output 223 of an amplifier222. The amplifier 222 may amplify the voltage across the Wheatstonebridge 240. The output voltage U_(out) may be read in processor 130through and A/D input 231.

To operate the thermal conductivity sensing element 211 at differenttemperatures at least one of the Wheatstone bridge resistors R₂₁, R₂₂,R₂₃ may be adjustable. For example, a fourth resistor R₂₄ may beprovided and selectively connected by a relay 233 in parallel to thefirst resistor R₂₁. By connecting a fourth resistor R₂₄ in parallel tothe first resistor R₂₁ the overall resistance which is in series withthe thermal conductivity sensing element decreases and the voltageacross the bridge changes. This ultimately leads to a change in theoutput voltage U_(out) of the amplifier 222 and a corresponding changein the temperature T₂ at which the thermal conductivity sensing element211 operates. The relay 233 may be controlled by the processor 130through an output 232.

The processor 130 may selectively open and close the relay 233 andthereby operate the thermal conductivity sensing element at a firsttemperature T₁ when the relay 233 is open and at a second temperature T₂when the relay 233 is closed. The processor 130 may evaluate the outputvoltage U_(out), to determine the thermal conductivity of therespiratory gas surrounding the thermal conductivity sensing element 211and ultimately determine the concentration of carbon dioxide therein bycomparing the thermal conductivity of the respiratory gas at the firsttemperature T₁ with its thermal conductivity at the second temperatureT₂.

An exemplary spiroergometer 300 which uses a respiratory gas sensingsystem is schematically shown in FIG. 3. The spiroergometer 300 has atube 301 having a proximal end 311. When in use, the proximal end 311 ofthe tube 301 may be connected to a breathing mask 312 which covers asubject's mouth and/or nose. The tube 301 has at least one distal end320. At least one thermal conductivity sensor 305 is provided within thetube 301. The thermal conductivity sensor 305 is operatively connectedto a processing module 350.

As illustrated, the tube 301 may split into a first branch 302 having afirst distal end 320 and a second branch 303 having a second distal end330. In this configuration, a first thermal conductivity sensor 305 maybe provided in the first branch 302 of the tube 301 and a second thermalconductivity sensor 335 may be provided in the second branch 303 of thetube 301. The second thermal conductivity sensor 335 may be operativelyconnected to the processing module 350.

Within the second branch 303 of the tube 301 one of more flaps 306,307may be provided to seal the second branch 303 and prevent air fromflowing through the second branch 303. The one or more flaps 306,307 mayinclude an entry flap 306 arranged close to a proximal end 313 of thesecond branch 303 where the tube 301 splits into the first branch 302and the second branch 303. Additionally or alternatively, an exit flap307 may be provided close to the distal end 330 of the second branch.The section of the second branch between the entry flap 306 and the exitflap 307 may form a trap for exhaled air: When both the entry flap 306and the exit flap 307 are closed while a subject is inhaling through thefirst branch 302 the previously exhaled air is trapped in the secondbranch 303 and can be analyzed while the exhaled air not moving.

The tube 301, including its first branch 302 and second branch 303,define a respiratory flow path. Respiratory gas flows through the tube301 when a subject is inhaling and exhaling. The flaps 306,307 in thesecond branch 303 may be passive and form a one-way valve. For example,the flaps 306,307 may open only when air is exhaled through the tube301, i.e. when air flows from the proximal end 311 to the distal ends320, 330. The unidirectional flow of air through the second branch 303is illustrated by the one-directional arrow 309 in the second branch303. As illustrated, no flaps are provided in the first branch 302 ofthe tube 301, allowing air to flow bidirectionally between the proximalend 311 of the tube 301 and the distal end 320 of the first branch 302.The bidirectional flow of the respiratory gas is illustrated by arrows308.

Alternatively, flaps 306, 307 may be actively controlled between an openstate and a closed state. In an active control configuration the entryflap 306 and/or the exit flap 307 may be operatively connected to theprocessing module 350. Whether actively controlled or passive, the flaps306,307 may be referred to as flow control valves.

FIG. 4 shows a more detailed schematic view of a thermal conductivitysensor 405 which may be used in the configuration as in FIG. 3. Thethermal conductivity sensor 405 is arranged inside a tube 401 andexposed to a respiratory gas 408 which is present in or flowing throughthe tube 401. The sensor 405 contains a first temperature sensingelement 413 which may be electrically connected to a first temperaturesensing circuit 371 within the processing module 350. The sensor 405further contains a second temperature sensing element 414 which may beelectrically connected to a second temperature sensing circuit 375within the processing module 350. The temperature sensing circuits371,375 may comprise A/D conversion components and power supplycomponents, e.g. a pull-up resistor and a measuring shunt. Thetemperature sensing circuits 371,375 are operatively connected to theprocessor 360 within the processing module 350. The temperature sensingelements 413, 414 may be arranged within the flow of respiratory air 408in the tube 401 inwardly of and radially spaced apart from thermalconductivity sensing elements 416,417,418. This arrangement providesthat the temperature sensing elements 413, 414 can measure thetemperature of respiratory air 408 flowing through the tube 401 withoutbeing affected by the actively heated thermal conductivity sensingelements 416, 417, 418. Furthermore, the first temperature sensingelement 413 may be arranged longitudinally spaced apart from the secondtemperature sensing element 414 with the thermal conductivity sensingelements 416,417,418 arranged in between. This arrangement guaranteesthat one temperature sensing element is arranged upstream and onetemperature sensing element is arranged downstream of the thermalconductivity sensing elements 416,417,418 regardless of the direction ofairflow 408 through the tube 401.

The sensor 405 uses one or more thermal conductivity sensing elements416,417,418. The thermal conductivity sensing elements 416,417,418 maybe arranged within a common plane, substantially parallel to the flow ofrespiratory air 408 in the tube 401. Alternatively, the thermalconductivity sensing elements 416,417,418 may be arranged in a planesubstantially perpendicular to the flow of air 408 in the tube 401.“Substantially parallel” should be understood to mean within 20 degrees,and more preferably within 10 degrees of being parallel. “Substantiallyperpendicular” should be understood to mean within 20 degrees of beingperpendicular, and more preferably within 10 degrees of beingperpendicular.

The thermal conductivity sensing elements 416,417,418 may bemeander-shaped resistive wires and operate as hot-wire anemometers. Thethermal conductivity sensing elements 416,417,418 may be made of nickel,platinum, molybdenum or tungsten. Use of another material with a highlinear resistance temperature coefficient is possible. The thermalconductivity sensing elements 416,417,418 may be made of the same ordifferent materials. Advantageously, the thermal conductivity sensingelements 416,417,418 are small and arranged close together, e.g. withinan area of 4 mm² to reduce power consumption and improve dynamicbehavior.

The thermal conductivity sensing elements 416,417,418 may be heated to apredetermined temperature by applying power to the sensing elements.More specifically, a variable voltage may be generated by sensorinterface circuitry (measuring circuit) 372,373,374 within theprocessing module 350. The sensor interface circuitry may include anamplifier and Wheatstone bridge configuration as shown in FIG. 1 or FIG.2. Each sensor interface circuit 372,373,374 may be operativelyconnected to the processor 360. Each sensor interface circuit372,373,374 may be capable of measuring the current (I) flowing throughthe associated thermal conductivity sensing element 416,417,418 andcommunicate the current (I) to the processor 360. The processor maycommunicate a voltage value (U) to each sensor interface circuit472,473,474 which in response thereto applies the voltage as determinedby the processor 360 to the associated thermal conductivity sensingelement 416,417,418.

The thermal conductivity sensing elements 416,417,418 may have atemperature-dependent electrical resistance, i.e. R=f(T). Consequently,the temperature of the thermal conductivity sensing elements 416,417,418may be controlled by applying a variable voltage (U) to a sensingelements and measuring the current (I) flowing through the sensingelement. In particular, the variable voltage (U) may be selected suchthat U=R_(target)* I, with R_(target) being the resistance of thethermal conductivity sensor at a desired temperature (T_(target)). Eachthermal conductivity sensing element may thus be operated at apredetermined temperature by regulating the voltage applied thereto.

The thermal conductivity sensing elements 416,417,418 may operate atthree different temperatures. Alternatively, the outer thermalconductivity sensing elements 416, 417 may operate at a firsttemperature and the inner thermal conductivity sensing element 418 mayoperate at a higher second temperature. This configuration provides abeneficial symmetrical temperature distribution across the sensor. Also,when used in an environment with bidirectional airflow, the outersensing elements operating at the same temperature may be used todetermine the direction of airflow: The downstream sensing element isexposed to air that is at a higher temperature due to being pre-heatedby the inner thermal conductivity sensing element 418. Consequently,less power is needed to maintain the first temperature of the downstreamthermal conductivity sensing element than of the upstream thermalconductivity sensing element.

As shown in FIG. 5, the voltage U which is required to heat a thermalconductivity sensing element to a given temperature increases with thetemperature of the thermal conductivity sensing element. Heating athermal conductivity sensing element to a higher temperature requires ahigher voltage. Independent of the gas which surrounds the thermalconductivity sensing element the curves 501,502,503 show that thevoltage (U) increases with the temperature (T).

FIG. 5 also shows that the voltage which is required to maintain a giventemperature of a thermal conductivity sensing element depends on the gaswhich surrounds the thermal conductivity sensing element. When thethermal conductivity sensing element is surrounded by oxygen (curve 501)or air (curve 502) a higher voltage is required than if the thermalconductivity sensing element is surrounded by carbon dioxide (curve503). Oxygen is a better thermal conductor than carbon dioxide.Consequently, more heating power and thus a higher voltage is requiredto compensate for heat losses from the thermal conductivity sensingelement into surrounding oxygen than into surrounding carbon dioxide. Inthis paper the voltage U that is required to maintain a particulartemperature of the thermal conductivity sensing element is considered tobe a measure of the thermal conductivity of the gas surrounding thesensing element. It should be understood that the voltage U also dependson the temperature of the gas surrounding the sensing element.

The thermal conductivity of a gas is largely independent of its pressureor density. The thermal conductivity of a gas does, however, depend onits temperature. The following table shows the thermal conductivity (λ)of various gases at different temperatures:

λ[W/mK] Gas @200 K @250 K @300 K @400 K @500 K Air 0.01836 0.022410.02623 0.03328 0.03971 Carbon 0.01295 0.01677 0.02515 0.03354 DioxideNitrogen 0.0187 0.0260 0.0326 0.0388 Hydrogen 0.1282 0.1561 0.182 0.2280.272 Oxygen 0.01848 0.02244 0.02615 0.03324 0.04010 Helium 0.118 0.1380.156 0.190 0.222 Argon 0.01245 0.01527 0.01787 0.02256 0.02675

The temperature-dependency of a gas's thermal conductivity can be use toinfer the composition of an unknown gas which surrounds a thermalconductivity sensing element. A method for measuring gas properties maytherefore heat a thermal conductivity sensing element to a predeterminedtemperature, measure the voltage that is necessary to maintain thepredetermined temperature, and infer the composition of the gassurrounding the thermal conductivity sensing element.

In practice, measuring thermal conductivity of a gas at a singletemperature is insufficient to determine the composition thereof. Asillustrated in FIG. 6, the voltage necessary to maintain a predeterminedtemperature of the thermal conductivity sensing element depends on thetemperature of the sensing element, on the composition of thesurrounding gas, and on the rate at which the gas is flowing past thesensing element. For example, if a voltage of 4V is necessary tomaintain a 200° C. operating temperature of the thermal conductivitysensing element, the sensing element might be surrounded by pure oxygenwhich is not moving or by a mixture of oxygen and carbon dioxide movingat 10 m/s past the sensor. Therefore, to determine the composition ofgas surrounding a thermal conductivity sensor, a single thermalconductivity sensing element may be operated at different temperaturesover time. Alternatively, more than one thermal conductivity sensingelement may be used, with different thermal conductivity sensingelements simultaneously operating at different temperatures.

A single thermal conductivity sensing element may for example beoperated at three different temperatures. First, the thermalconductivity sensing element may be operated at 100° C. and the voltageU₁₀₀ that is required to maintain the 100° C. operating temperature maybe determined and stored in a processor. Next, the conductivity sensingelement may be operated at 200° C. and the voltage U₂₀₀ that is requiredto maintain the 200° C. operating temperature may be determined andstored in the processor. Finally, the conductivity sensing element maybe operated at 300° C. and the voltage U₃₀₀ that is required to maintainthe 300° C. operating temperature may be determined and stored in theprocessor.

Alternatively, a thermal conductivity sensor may comprise a firstthermal conductivity sensing element 416, a second thermal conductivitysensing element 417, and a third thermal conductivity sensing element418. The first thermal conductivity sensing element 416 may operate at afirst predetermined temperature, e.g. 100° C. The second thermalconductivity sensing element 417 may operate at a second predeterminedtemperature, e.g. 200° C. Lastly, the third thermal conductivity sensingelement 418 may operate at a third predetermined temperature, e.g. 300°C.

The processor 130,360 may determine the composition and/or the rate ofgas flowing through the tube in response to the values U₁₀₀, U₂₀₀ andU₃₀₀. The processor may determine the composition and/or rate of gasthrough look-up tables that are stored in a memory component 133.

When used in spiroergometry, the processor may determine the amount ofcarbon dioxide in exhaled air and the total volume and/or mass of airthat is passing through a tube. The processor may determine thedirection of air flow to distinguish between inhaled and exhaled air.

A thermal conductivity sensor 405 that uses two or more thermalconductivity sensing element 416,417,418 which are simultaneouslyoperating a different temperatures has some inherent advantages over asingle thermal conductivity sensing element 211 which is operated atdifferent temperatures over time. Primarily, the multi-element sensor110,405 has better dynamic behavior, avoiding the delay that is inherentto a single-element sensor 210 which requires time for heating andcooling the sensing element 211 to different temperatures. The thermalconductivity sensing elements 416,417,418 in a multi-element sensor 405may preferably be arranged close to each other so that the sensingelements are thermally coupled to each other, thereby reducing theoverall power requirements for the sensor 405. Reduced power consumptionis particularly advantageous when the sensor is used in battery-powered,mobile devices.

Within a multi-sensing element sensor 405 the third thermal conductivitysensing element 418 may be arranged centrally between the first thermalconductivity sensing element 416 and the second thermal conductivitysensing element 417. In such an arrangement the centrally arranged thirdthermal conductivity sensing element 418 is preferably operated at thehighest temperature of the three sensing elements. The thermalconductivity sensing elements 416,417 that are arranged on the outsideare operated at lower temperatures, thereby reducing the overall powerconsumption of the sensor 405.

Various uses of the disclosed sensor and method for measuring gasproperties are envisioned. In the field of spiroergometry systems, thesensor 405 may for example be used within a breathing mask that a personcan wear while exercising. In the medical field, the sensor and methodmay be applied to monitoring the flow and composition of air with ananesthetic gas during surgery. The sensor and method may be used beyondthe medical field, e.g. in industrial or consumer applications.

Both the single-sensing element variant and the multi-sensing elementvariant of the thermal conductivity sensor can measure the gas mass flowand the gas composition of air or another mixture of gases. The methodcan be used to derive a gas volume flow from the gas mass flow, even ifthe gas changes its composition during the measurement. This isnecessary since different gases occupy a different volume with the samemass. In medical technology, for example, the same sensor can then beused to calculate the volume of air during inspiration and duringexpiration, even though the CO₂ content is significantly higher duringexpiration than during inspiration. That is, even though the mass flowof air during inspiration and expiration may be different, the correctvolume flow can be derived by considering the measured increase in CO₂in the exhaled air.

The operating principle of the sensor is based on hot-wire anemometry:An electrical conductor (wire, hot wire) is heated during hot wireanemometry. If a colder gas flows past the heated conductor, theconductor is either cooled or requires more power to maintain the sametemperature. The heated conductor may be operated at a predeterminedtemperature by adjusting the power that is applied to the conductor. Thepower may be adjusted by controlling the voltage applied to theconductor through a variable voltage source. Alternatively, the powerapplied to the heated conductor may be kept constant, and the wire'stemperature may be determined by measuring its resistance. The powerrequired to maintain a constant temperature of the wire depends on themass flow of gas passing by the wire.

The thermal conductivity sensing elements 416,417,418 may operate atpredetermined absolute temperatures, e.g. 120° C., 220° C., and 320° C.Alternatively, the thermal conductivity sensing elements 416,417,418 mayoperate at a predetermined temperature difference above the temperatureof the gas that is to be analyzed. Utilizing the temperature sensingelements 413, 414 the processor 360 may determine the temperatureT_(gas) of gas 408 inside the tube 401. The processor 360 may thenselect a target temperature of the thermal conductivity sensing elements416,417,418 by applying a predetermined temperature differential ΔT tothe temperature of the gas. For example, the processor 360 may selectthe target temperature of the thermal conductivity sensing elements416,417,418 to be 100° C., 200° C. and 300° C. above the temperature ofthe gas 408.

In yet another variation the processor may apply predetermined amountsof heating power to the different thermal conductivity sensing elements416,417,418, which will then assume a variable temperature above thetemperature of the gas 408 within the tube 401.

It is possible to measure a gas concentration by measuring its thermalconductivity. Gases (e.g. CO₂) or gas mixtures (e.g. air) have specificthermal conductivities at given gas (air) temperatures. Thermalconductivity sensing elements 416,417,418 can be used to determine thethermal conductivity of a gas. The amount of heat that is transferredfrom the thermal conductivity sensing elements 416,417,418 into the gasdepends on the gas's temperature, the gas's heat capacity, theinterfacial thermal resistance between the sensing elements and the gas,and other properties of the gas or the sensor. The concentration of oneor more gases within an unknown mixture of gases may thus be inferred byobserving the thermal conductivity of the unknown mixture of gases atdifferent temperatures and comparing the observed thermal conductivitieswith the thermal conductivities of gases with known composition at thesame temperatures.

The electrical resistance of a heated wire increases with thetemperature of the wire. Consequently, the voltage applied to the heatedwire is correlated to the output power. In the following description thevoltage applied to the heated wire is used to indicate the power whichis consumed by the heated wire, given the known relationship of thewire's resistance and temperature. The processing module 120 regulatesthe hot wire by applying a variable voltage so that the wire maintainsthe desired temperature (resistance). The voltage required to maintain agiven wire temperature is thus a measure of the heat transfer, the heatcapacity and other properties of the gas since the resistance of thewire is kept constant by the processing module 120. The output power ofthe wire is P=U²/R.

FIG. 5 shows that the voltage at the wire depends on the selected sensortemperature and on the composition of the gas. For each gas there is adefined temperature range. At the same sensor temperature, the voltagefor each gas mixture is constant and thus unambiguous. Morespecifically, FIG. 5 shows the relationship of the voltage U [V] overthe sensor temperature in degrees Celsius with different gases. The gastemperature here is 20 degrees Celsius. Each gas composition shows adifferent curve 501, 502, 503. This is a form of temperaturespectroscopy. If the sensor temperature is known, the type of gas can bedetermined. The prerequisite for this is that it is precisely knownwhich gas mixture components can change. This is often sufficient forbiomedical applications because the application only considers a changein the CO2 or oxygen content.

Depending on the gas composition, the thermal conductivity sensingelements require different amount of power to maintain their targettemperatures. The gas mass flow past the sensing elements also carriesenergy away from the sensor. The question therefore arises as to whetherthe sensor voltage reacts to a change in the gas mass flow or to achange in the gas composition. This question can be answerd by operatingthe sensors at two different temperatures.

Referring to FIG. 6, we assume that a voltage of 3.0 volts is applied toa sensing element to reach an operating temperature of 50 degreesCelsius above the temperature of the surrounding gas. This mightindicate that the sensing element is surrounded by pure oxygen (withoutany movement) or flowing air at a speed of 10 meters per second. Furthercombinations would be conceivable, for example, that it is very fast(more than 10 meters per second) flowing CO2. If, however, a secondsensing element is used which operates at 300 degrees Celsius above thetemperature of the gas, pure oxygen will require a voltage of 4.5 voltsto be applied to the second sensing element while air at a flow rate of10 m/s will require a voltage of 4.8 volts to be applied to the secondsensing element.

Preferably, several sensing elements 416,417,418 are disposed on amembrane 415 and operated at different temperatures in order to increasethe accuracy and dynamic behavior of the sensor 405. This allows thesimultaneous determination of mass flow and composition of a gas. Forexample, the mass flow of air and the concentration of CO₂ in air can besimultaneously measured with a multi-sensing-element sensor 405.

A multi-sensing element sensor 405 will use at least two sensingelements operating at two different temperatures, but may have more thantwo sensing elements, each operating at a different temperature. Forexample, a multi-sensing element sensor 405 may utilize three, four,five or six sensing elements. More generally speaking, a sensor 405 mayutilize n sensing elements operating at n temperatures.

One method for measuring the thermal conductivity of a gas is thedetermination of how much power can be delivered to the gas from aheating element. The power required to maintain a set temperature of aheating element is positively correlated with the thermal conductivityof the surrounding gas. A reduction in the power required to maintainthe heater temperature indicates a reduced thermal conductivity of thesurrounding gas. This method can be used, if the temperature of the gasto be analyzed remains constant. If the temperature of the gas aroundthe heating element is not constant, a method for compensating for achanging ambient temperature is needed.

A first method may use a static operating temperature, measuretemperature of the gas, and mathematically correct the measured value.In this method, a heating element is maintained at a constant operatingtemperature independent of ambient temperature. The ambient temperatureof the gas to be analyzed is measured by a separate independenttemperature sensor. The ambient temperature can subsequently be used tomathematically correct the measured value based on the power absorbed bythe sensor.

In an alternative second method the influence of ambient temperature canbe eliminated by varying the operating temperature of the sensor. Forthis purpose, the sensor is not kept at a constant operatingtemperature, as described above, but is varied as a function of theambient temperature. That is, the warmer the gas to be analyzed, thehotter the operating temperature of the thermal conductivity sensor. Inthis case the difference between the operating temperature and theambient temperature may be kept constant. The power introduced into thesensor can thus be used directly for a measurement of the thermalconductivity of the gas.

Thermal conductivity measurements may utilize different sensing elementtemperatures with consideration of temperature-dependent thermalconductivity. A predetermined constant operating temperature of thesensing element may be used, the temperature of the gas may be measuredand the measured value may be mathematically corrected. The mathematicalcorrection takes into account that the thermal conductivity changes withthe temperature. In practice, the calculation may utilize lookup-tables.The temperature-dependent thermal conductivity may automatically betaken into account by calibrating the sensor system at differenttemperatures to the different gas concentrations.

Alternatively, the temperature difference between the operatingtemperature of the sensing element and the ambient temperature of thesurrounding gas may be kept constant. Thus, changes in the measuredpower required by the heating element to maintain this temperaturedifference depend solely on the change in concentration of the gas to beexamined. In contrast to the above-described case, where the measuredpower can now be used alone for determining the thermal conductivity,the modified thermal conductivity is taken into account in this case bycalibrating the system at different ambient temperatures. By means of anindependent temperature sensor for measuring the gas temperature, themeasured power can be assigned to the corresponding thermal conductivityat the same gas temperature.

For example, to measure the CO₂ concentration in air, a first sensingelement may be operated at 50° C. above the air temperature and thesecond sensor may be operated at 200° C. above air temperature. If thisdelta-temperature is compensated to the extent that the required powerof the heater remains the same at the same CO₂ concentration, the twosensors will always have the same output signal if the CO₂ concentrationremains the same. If, however, CO₂ concentration in the air changes, thetwo output signals will differ.

To achieve more accurate measurements, a sensor may utilize not only twosensing elements at different delta-temperatures but a number of nsensing elements operating at n different delta temperatures. Thereby, aspectroscopy on the conductivity of the gases can be performed and thegas composition can be spectroscopically determined. As before, thesensing elements may operate at predetermined constant absolutetemperatures or at predetermined delta temperatures above the ambienttemperature of the gas that is to be analyzed.

To achieve even more accurate measurements of the CO₂ concentration inair it is desirable to eliminate the influence of air movement while theair is being analyzed. The second branch 303 of the tube 301 as shown inFIG. 3 utilizes two flaps 306,307 which can be selectively closed suchthat air can no longer flow through the second branch 303 of tube 301.It is for example possible to open the flaps 306,307 during exhaling andclose them during inhaling. Exhaled air is thus trapped between theentry flap 306 and the exit flap 307 and can be accurately analyzedwithout having to account for the influence of air movement on the powerrequired to maintain a particular sensing element's temperature. In thisoperating mode it is sufficient to determine the volume of air flowingthrough the tube 301 during inhaling, since the exhaled volume mustequal the inhaled volume over longer time periods.

The disclosed method measures the thermal conductivity of gases bydetermining the power that can be delivered into the gas by a heatingelement. The method may use a heating element which is maintained at aconstant absolute temperature and the power required for it is a measureof thermal conductivity. For this method to function at different gastemperatures, a constant, absolute temperature is not used, but aconstant overtemperature, i.e. a constant temperature difference betweenthe gas temperature and the temperature of the heating element, isapplied. Thus, the applied power can be correlated to the thermalconductivity. To infer from the thermal conductivity to the gascomposition, it is assumed that the same power is necessary to heat aheating element by relative e.g. 20 degrees Celsius. However, thisassumption is not always correct, since the thermal conductivity of agas is not the same at all temperatures. This knowledge can be used intwo ways.

Either, one complicates the relative temperature control in such a waythat it adjusts to a slightly adjusted (compensated) relativetemperature. This is selected in such a way that the relativetemperature is adjusted so as to compensate for the deviation of thechange in the thermal conductivity over the temperature of the gas to bemeasured. ΔT=const (relative temperature of the heater)

(T _(heater) −T _(gas))=ΔT

If the thermal conductivity coefficient change over the temperature isnot to be taken into account one may assume T_(compensation)=0.Otherwise, T_(compensation) is the temperature which must be subtractedor added to ΔT so that the same power is used at the same gasconcentration even though the coefficient of thermal conductivitychanges due to a change in the gas temperature:

ΔT−T _(compensation) =ΔT _(compensated)

In the case of several overtemperatures, one measures the thermalconductivity coefficient of gases in different temperature dependence inorder to increase the selectivity of the sensor system. For thispurpose, one can use several sensors and operate them at differentovertemperatures, or one uses a sensor at different temperatures overtime.

The smaller the heating element, the faster the sensor system reacts tochanges in the gas concentration. An arrangement of the sensors on acircular envelope 422 (envelope circle) leads to the best result.Leaking heat via the contact points or the carrier medium of the heatercan lead to the outflow of energy levels which are not related to thegas concentration. For this reason, the sensors should be very small andthe electrical feed 412 should be designed in such a way that as littleheat as possible is dissipated through them. Preferably, the membrane415 should have more than twice the diameter as the imaginary envelopingcircle 422. The diaphragm distance 421 to the enveloping circle 422should also have at least half the diameter of the imaginary envelopingcircle 422. The classic design is a thin wire, which is stretched on twothin contact needles. A further structure is the planar construction inwhich a thin-film structure of nickel, platinum, molybdenum, etc. isapplied in the form of a heating coil, spiral-shaped or meander-shaped,etc., on a substrate such as glass, ceramic, silicon, polyimide, plasticetc.

An alternative application in which the sensor may be advantageouslyused is a breathing training device 700 as shown in FIG. 7. Thebreathing training device 700 may be used for respiratory muscletraining and enable a patient to better control his or her breathing.The breathing training device 700 may, for example, be used withpatients suffering from chronic obstructive pulmonary disease(COPD).COPD is a type of obstructive lung disease characterized bylong-term poor airflow. The main symptoms include shortness of breathand cough with sputum production. Breathing training, i.e. the targetedstrengthening of the respiratory muscular system, has been found to bean effective treatment option for COPD, leading to significantimprovement of patient's stamina and quality of life without the use ofcortisone drugs.

The breathing training device 700 contains a mouthpiece 712 which isattached to a proximal end 711 of a tube 701. The tube 701 forms thebody of the breathing training device 700. The tube 701 branches into afirst branch 702 and a second branch 703. The breathing device 700 maybe connected to a processing module 750. The processing module 750 maybe integral to the breathing training device 700.

A distal end 720 of the first branch may be open. A first flow controlvalve 721 may be provided in the first branch 702 of the breathingtraining device 700 and operatively connected to the processing module750. A first thermal conductivity sensor 725 may be provided in thefirst branch 702 of the breathing training device 700. The first thermalconductivity sensor 725 may be operatively connected to the processingmodule 750. By reading the first thermal conductivity sensor 725 theprocessing module 750 may determine the oxygen and carbon dioxideconcentration of air flowing through the first branch 702.

A second thermal conductivity sensor 735 may be provided in the secondbranch 703 of the breathing training device 700. The second sensor 735may be operatively connected to the processing module 750. By readingthe second sensor 735 the processing module 750 may determine the oxygenand carbon dioxide concentration of air flowing through the secondbranch 703. The first sensor 725 and the second sensor 735 may comprisethermal conductivity sensing elements as described before. The firstsensor 725 and/or the second sensor 735 may additionally comprise apressure sensing element.

Installed at the distal end 730 of the second branch 703 may be anelastic bag or reservoir 731. A second flow control valve 706 may bearranged at an inlet (proximal) end 713 of the second branch 703.

While in use, the first flow control valve 721 and the second flowcontrol valve 706 may be selectively adjusted between an open positionand a closed position. The flow control valves 721,706 may be adjustablein multiple steps between the open position and the closed position,allowing for intermediate partially opened states in which air flowthrough the respective flow control valve is restricted, but notcompletely cut off. The flow control valves 721,706 may be selectivelyopened and closed in response to the pressure and/or flow of air throughthe first branch 702 and the second branch 703 as determined by theprocessing module 750 by reading the first thermal conductivity sensor725 and the second thermal conductivity sensor 735. For example, in oneparticular operating condition the second flow control valve 706 may becompletely closed, so that the second branch 703 is shut. The first flowcontrol valve 721 may be partially opened to provide a desiredresistance to inhaling and/or exhaling of air through the tube 701 andthe first branch 702 as dictated by a training protocol that isprogrammed into a memory component 771 in the processing module 750.

The training protocol within processing module 750 may determine theopening and closing of flow control valves 721,706 in the breathingtraining device 700. The degree, to which the flow control valves721,706 are opened, in turn determines the pneumatic resistance of air708 flowing through the tube 701. The pneumatic resistance may beselectively adjusted separately during inhaling and exhaling. Thepneumatic resistance during inhaling and exhaling may be selected basedon previously conducted tests, and selectively adjusted through a userinterface. The pneumatic resistance may be configured to change over thecourse of an exercise. Data related to air flow and concentration ofoxygen and carbon dioxide over time may be stored in a non-volatilememory component 771 in the processing module 750 for further analysis.

During use, a patient may start breathing heavier than usual, and beginhyperventilating. Hyperventilation occurs when the rate and quantity ofalveolar ventilation of carbon dioxide exceeds the body's production ofcarbon dioxide. Hyperventilation may cause physical symptoms such asdizziness, tingling in the lips, hands or feet, headache, weakness,fainting and seizures. In extreme cases it can cause carpopedal spasms(flapping and contraction of the hands and feet). By monitoring theconcentration of oxygen and volume of inhaled air, through use of thefirst thermal conductivity sensor 725, the processing module 750 canrecognize this condition, and actively prevent it. To preventhyperventilation the processing module 750 may open the second branch703 by selectively adjusting the second flow control valve 706, so thatat least a part of the patient's exhaled air is captured in the elasticreservoir 731. The exhaled air in reservoir 731 contains significantlymore carbon dioxide than normal air. The concentration of carbon dioxidein the exhaled air can be determined through the second thermalconductivity sensor 735. While the patient is inhaling the processingmodule 750 may open, at least partially, the second branch 703 so thatthe patient inhales a mixture of fresh air 708 through the first branch702 and previously exhaled air 709 through the second branch 703. Byusing the flow measuring capability of the first sensor 725 and thesecond sensor 735 the processing module 750 can adjust the concentrationand/or volume of carbon dioxide that the patient inhales and activelyprevent hyperventilation.

The processing module 750 may comprise a processor 760 which isoperatively connected to a memory 771, analog inputs 772, analog outputs773, a user interface 774 and an external interface 775.

Exemplary Embodiments

A sensing system for measuring a gas property, comprising:

-   -   a thermal conductivity sensing element disposed within a mixture        of gases; and    -   a processing module operatively connected to the thermal        conductivity sensing element,    -   wherein the processing module applies power to alternately heat        the thermal conductivity sensing element to a first temperature        and to a second temperature which is different than the first        temperature and    -   wherein the processing module measures a first thermal        conductivity of the mixture of gases while the thermal        conductivity sensing elements is heated to the first temperature        and    -   wherein the processing module measures a second thermal        conductivity of the mixture of gases while the thermal        conductivity sensing element is heated to the second temperature        and    -   wherein the processing module determines a concentration of a        first gas within the mixture of gases in response to the        measured first thermal conductivity and the measured second        thermal conductivity.

A sensing system for measuring a gas property, comprising:

-   -   a first thermal conductivity sensing element disposed within a        mixture of gases; a second thermal conductivity sensing element        disposed within the mixture of gases; and    -   a processing module operatively connected to the first thermal        conductivity sensing element and to the second thermal        conductivity sensing element,    -   wherein the processing module measures a first thermal        conductivity of the mixture of gases while the first thermal        conductivity sensing element is heated to a first operating        temperature and    -   wherein the processing module measures a second thermal        conductivity of the mixture of gases while the second thermal        conductivity sensing element is heated to a second operating        temperature and    -   wherein the processing module determines a concentration of a        first gas within the mixture of gases in response to the        measured first thermal conductivity and the measured second        thermal conductivity.

A device for analyzing a breathing gas, comprising:

a first thermal conductivity sensing element disposed within thebreathing gas, the first thermal conductivity sensing element beingoperatively connected to a processing module,

-   -   wherein the processing module measures a first thermal        conductivity of breathing gas flow while the first thermal        conductivity sensing element is heated to a first operating        temperature and    -   wherein the processing module measures a second thermal        conductivity of the breathing gas flow while the first thermal        conductivity sensing element is heated to a second operating        temperature and    -   wherein the processing module determines a concentration of        carbon dioxide within the breathing gas flow in response to the        measured first thermal conductivity and the measured second        thermal conductivity.

A breathing training apparatus, comprising:

-   -   a tube having a proximal end connected to a breathing mask and a        distal end connected to a splitter;    -   a first branch having a proximal end connected to the splitter        and an open distal end; a first thermal conductivity sensing        element arranged within the first branch and operatively        connected to a processing module;    -   a first flow control valve arranged in the first branch and        operatively connected to the processing module;    -   a second branch having a proximal end connected to the splitter        and a distal end connected to an inflatable reservoir;    -   a second thermal conductivity sensing element arranged within        the second branch and operatively connected to a processing        module;    -   a second flow control valve arranged in the second branch and        operatively connected to the processing module.

The breathing training apparatus as above,

-   -   wherein the control apparatus determines a concentration of        carbon dioxide in exhaled air flowing through the tube and    -   controls the first flow control valve to reduce the flow of air        through the first branch and    -   controls the second flow control valve to increase the flow of        air through the second branch.

A method for measuring a property of a mixture of gases, comprising:

-   -   providing one or more thermal conductivity sensing elements        within the mixture of gases;    -   applying heating power to one of the one or more thermal        conductivity sensing elements and controlling the heating power        to maintain a selected first temperature;    -   measuring a first voltage and/or first power required to        maintain the first temperature;    -   applying heating power to one of the one or more thermal        conductivity sensing elements and controlling the heating power        to maintain a selected second temperature;    -   measuring a second voltage and/or second power required to        maintain the second temperature;    -   determining the concentration of at least one gas contained in        the mixture of gases in response to the measured first voltage        and/or first power and the measured second voltage and/or second        power.

While the present invention has been described with reference toexemplary embodiments, it will be readily apparent to those skilled inthe art that the invention is not limited to the disclosed orillustrated embodiments but, on the contrary, is intended to covernumerous other modifications, substitutions, variations and broadequivalent arrangements that are included within the spirit and scope ofthe following claims.

What is claimed is:
 1. A respiratory gas sensing system, comprising: athermal conductivity sensor disposed within a respiratory flow path; anda processing module operatively connected to the thermal conductivitysensor, wherein the processing module measures a first thermalconductivity of a respiratory gas at a first temperature and wherein theprocessing module measures a second thermal conductivity of therespiratory gas at a second temperature, the second temperature beinghigher than the first temperature, and wherein the processing moduledetermines a concentration of carbon dioxide within the respiratory gasin response to the measured first thermal conductivity and the measuredsecond thermal conductivity.
 2. The respiratory gas sensing system as inclaim 1, wherein the thermal conductivity sensor comprises: two outerthermal conductivity sensing elements; and an inner thermal conductivitysensing element arranged between the two outer thermal conductivitysensing elements, wherein the processing module is configured to operatethe two outer thermal conductivity sensing element at the firsttemperature and to operate the inner thermal conductivity sensingelement at the second temperature.
 3. The respiratory gas sensing systemas in claim 1, wherein the thermal conductivity sensor comprises: afirst thermal conductivity sensing element; and a second thermalconductivity sensing element, and wherein the processing module isconfigured to operate the first thermal conductivity sensing element atthe first temperature and to operate the second thermal conductivitysensing element at the second temperature.
 4. The respiratory gassensing system as in claim 3 arranged within a tube through which therespiratory gas flows, wherein the first thermal conductivity sensingelement and the second thermal conductivity sensing element are arrangedin a plane substantially parallel to a longitudinal extension of thetube.
 5. The respiratory gas sensing system as in claim 3, wherein thefirst thermal conductivity sensing element is a first meander-shapedresistive wire forming part of a first measuring circuit and wherein thesecond thermal conductivity sensing element is a second meander shapedresistive wire forming part of a second measuring circuit.
 6. Therespiratory gas sensing system as in claim 3, wherein the first and thesecond thermal conductivity sensing elements are arranged on a membranewithin an area of less than 4 mm².
 7. The respiratory gas sensing systemas in claim 1, wherein the thermal conductivity sensor comprises athermal conductivity sensing element and wherein the processing moduleis configured to alternately operate the thermal conductivity sensingelement at the first temperature and at the second temperature.
 8. Therespiratory gas sensing system as in claim 1, wherein the thermalconductivity sensor comprises: two outer thermal conductivity sensingelements; two inner thermal conductivity sensing elements arrangedbetween the two outer thermal conductivity sensing elements; and acentral thermal conductivity sensing element arranged between the twoinner thermal conductivity sensing elements, wherein the processingmodule is configured to operate the two outer thermal conductivitysensing element at the first temperature and to operate the two innerthermal conductivity sensing element at the second temperature and tooperate the central thermal conductivity sensing element at a thirdtemperature which is higher than the second temperature.
 9. Therespiratory gas sensing system as in claim 1, wherein the processingmodule comprises a memory having data stored in a lookup-table andwherein the processing module determines the concentration of carbondioxide by reference to the lookup-table, wherein the lookup-tablecomprises predetermined values associating the concentration of carbondioxide with thermal conductivity measurements.
 10. The respiratory gassensing system as in claim 1, wherein the thermal conductivity sensorcomprises: a first thermal conductivity sensing element; a secondthermal conductivity sensing element; and a third thermal conductivitysensing element, and wherein the processing module is configured tooperate the first thermal conductivity sensing element at the firsttemperature and to operate the second thermal conductivity sensingelement at the second temperature and to operate the third thermalconductivity sensing element at a third temperature, the thirdtemperature being higher than the second temperature.
 11. Therespiratory gas sensing system as in claim 10, wherein the third thermalconductivity sensing element is arranged centrally between the firstthermal conductivity sensing element and the second thermal conductivitysensing element.
 12. The respiratory gas sensing system as in claim 1,wherein the first temperature and the second temperature arepredetermined absolute temperatures.
 13. The respiratory gas sensingsystem as in claim 1, wherein the first temperature is selected inresponse to measuring a temperature of the respiratory gas.
 14. Aspiroergometer, comprising: a tube having a proximal end and a distalend; a breathing mask connected to the proximal end of the tube; and therespiratory gas sensing system as in claim 1, wherein the thermalconductivity sensor is arranged within the tube.
 15. The spiroergometeras in claim 14, wherein the breathing mask and the respiratory gassensing system are formed as a portable, battery powered device which isworn by a subject while exercising.
 16. The spiroergometer as in claim14, wherein the tube comprises a first branch and a second branch andwherein a trap section for exhaled respiratory gas is formed between twoflow control valves in the second branch, and wherein the thermalconductivity sensor is arranged within the trap section of the secondbranch.
 17. A respiratory flow control system, comprising: a tube havinga proximal end and a distal end; a breathing mask connected to theproximal end of the tube; and the respiratory gas sensing system as inclaim 1, wherein the tube comprises a first branch and a second branch,a flow of respiratory gas through the first branch being controllable bya first flow control valve arranged within the first branch and a flowof respiratory gas through the second branch being controllable by asecond flow control valve arranged within the second branch, wherein thefirst flow control valve and the second flow control valve arecontrolled by the processing module in response to the concentration ofcarbon dioxide determined by the processing module.
 18. The respiratoryflow control system as in claim 17, wherein the second branch has adistal end connected to a reservoir comprising an anesthetic gas. 19.The respiratory flow control system as in claim 17, wherein theprocessing module determines a respiratory volume flow in response tomeasuring a respiratory mass flow and the concentration of carbondioxide.
 20. A method for analyzing a respiratory gas, comprising:providing a thermal conductivity sensor within a respiratory flow path;measuring a first thermal conductivity of the respiratory gas at a firsttemperature; measuring a second thermal conductivity of the respiratorygas at a second temperature; and determining a concentration of carbondioxide within the respiratory gas in response to the measured firstthermal conductivity and the measured second thermal conductivity. 21.The method as in claim 20, wherein determining the concentration ofcarbon dioxide within the respiratory gas comprises comparing themeasured first thermal conductivity and the measured second conductivitywith known thermal conductivities of gases at different temperatures.22. The method as in claim 20, wherein the thermal conductivity sensoris provided within a trap for exhaled air and wherein the first thermalconductivity and the second thermal conductivity are measured whileexhaled air is trapped.